Membrane bioreactor (MBR) systems have become popular in wastewater treatment. MBR systems typically include one or more biological reactors, such as anaerobic, anoxic, and aerobic reactors, followed by one or more membrane tanks. Each membrane tank includes one or more membrane modules. A permeate pump creates a low pressure in the membrane modules and causes wastewater to be induced into the membranes. In the process, the membranes filter and reject contaminants such as suspended solids and produce a permeate.
It is expected that the trend towards utilizing membrane filters will increase as membrane costs decrease. Generally, MBR systems allow activated sludge processes to operate at a significantly higher MLSS concentration than with conventional clarification processes. As such, MBR systems eliminate the need for secondary clarifiers for liquid and solid separation. In addition to those advantages, MBR systems typically are constructed on a smaller footprint, and in the end, provides superior treated water quality.
The use of membranes in biological treatment is not without its drawbacks. One of the major concerns in utilizing submerged membranes is that they tend to foul and to provide continuous cleaning there is often provided air scouring. Air scouring results in a significant operating cost.
More particularly, membrane modules are submerged in a bioreactor and mixed liquor is suctioned through the membrane as permeate. As noted above, air scouring is provided below the membrane module and generates a cross flow movement across the membrane surface. This cross flow movement tends to clean the membrane and sustain permeation. Furthermore, it is typical to operate the membranes in on and off cycles. A cycle generally includes a permeation phase and a relaxation phase. The relaxation phase occurs when the permeate pump or pumps are shut off. During the relaxation phase, air scouring continues while permeation is suspended.
There are a number of process variables that are interrelated and impact the successful operation of an MBR system. The permeate flux determines the rate of transport of colloid and suspended solids towards the membrane surface. Because membrane filtration retains colloid and suspended solids, permeation results in concentration polarization (CP) and cake layer build up on the membrane surface. The higher the permeate flux, the faster the CP/cake layer will build up onto membrane surfaces. Both CP and cake layer limit the permeate flux in a constant pressure operation or result in an increase in transmembrane pressure (TMP) in a constant permeate flux operation.
Air scouring of the membranes induces a cross flow movement of water and/or shear force across the surface of the membrane, which increases the mass transfer of the retained colloids and suspended solids away from the membrane surface. In theory, suspended solids should not build up on a membrane surface if the convective flux of solids towards the membrane caused by permeation is less than the back transport of solids away from the membrane caused by the cross flow effect of air scouring. Therefore, while air scouring is effective, excess air scouring beyond a certain point has no positive affect on the reduction of CP and cake layer.
Scouring air provides a means to not only to minimize CP and the thickness of a sludge layer during a permeation phase, but also to clean the membrane surfaces during relaxation phases. If the membrane surfaces cannot be cleaned within each permeation-relaxation cycle, the cake layer will continue to deposit onto the membrane surface. This will lead to a rapid increase in TMP in a constant permeate flux operation or a rapid decrease in permeate flux in a constant pressure operation.
Generally, the greater the CP and cake layer build up, the greater is the degree of fouling because both phenomena increase the contact between the membrane surface and the fouling material. Membrane fouling will lead to an increase in the frequency of chemical-in-place (CIP) cleaning (chemical cleaning), thereby leading to more chemical consumption, less throughput in permeation phases, and shorter life expectancy of the membranes.